Hello, James.
Thank you for the meticulous test and review.
On Fri, 2017-08-11 at 18:02 +0100, James Morse wrote:
> Hi Hoeun,
>
> On 07/08/17 06:09, Hoeun Ryu wrote:
> >
> > Commit 0ee5941 : (x86/panic: replace smp_send_stop() with kdump friendly
> > version in panic path) introduced crash_smp_send_stop() which is a weak
> > function and can be overriden by architecture codes to fix the side effect
> (overridden)
It'll be fixed in the next version.
>
>
> >
> > caused by commit f06e515 : (kernel/panic.c: add "crash_kexec_post_
> > notifiers" option).
> >
> > ARM64 architecture uses the weak version function and the problem is that
> > the weak function simply calls smp_send_stop() which makes other CPUs
> > offline and takes away the chance to save crash information for nonpanic
> > CPUs in machine_crash_shutdown() when crash_kexec_post_notifiers kernel
> > option is enabled.
> >
> > Calling smp_send_crash_stop() in machine_crash_shutdown() is useless
> > because all nonpanic CPUs are already offline by smp_send_stop() in this
> > case and smp_send_crash_stop() only works against online CPUs.
>
> >
> > The result is that /proc/vmcore is not available with the error messages;
> > "Warning: Zero PT_NOTE entries found", "Kdump: vmcore not initialized".
> When I tried this I got one of these warnings for each secondary CPU, but the
> vmcore file was still available. When I ran 'crash' on the vmcore it reported:
> >
> > CPUS: 6 [OFFLINE: 5]
> Did I miss as step to reproduce this? If not, can we change this paragraph to
> say something like:
> >
> > The result is that secondary CPUs registers are not saved by
> > crash_save_cpu()
> > and the vmcore file misreports these CPUs as being offline.
Actually the commit log comes from the patch to fix a similar issue in arm port.
I'll change the commit log with yours.
>
> >
> > crash_smp_send_stop() is implemented to fix this problem by replacing the
> > exising smp_send_crash_stop() and adding a check for multiple calling to
> (existing)
It'll be fixed in the next version.
>
>
> >
> > the function. The function (strong symbol version) saves crash information
> > for nonpanic CPUs and machine_crash_shutdown() tries to save crash
> > information for nonpanic CPUs only when crash_kexec_post_notifiers kernel
> > option is disabled.
> >
> > * crash_kexec_post_notifiers : false
> >
> > panic()
> > __crash_kexec()
> > machine_crash_shutdown()
> > crash_smp_send_stop() <= save crash dump for nonpanic cores
> >
> > * crash_kexec_post_notifiers : true
> >
> > panic()
> > crash_smp_send_stop() <= save crash dump for nonpanic cores
> > __crash_kexec()
> > machine_crash_shutdown()
> > crash_smp_send_stop() <= just return.
>
> >
> > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> > index dc66e6e..73d8f5e 100644
> > --- a/arch/arm64/kernel/smp.c
> > +++ b/arch/arm64/kernel/smp.c
> > @@ -977,11 +977,21 @@ void smp_send_stop(void)
> > }
> >
> > #ifdef CONFIG_KEXEC_CORE
> > -void smp_send_crash_stop(void)
> > +void crash_smp_send_stop(void)
> > {
> > + static int cpus_stopped;
> > cpumask_t mask;
> > unsigned long timeout;
> >
> > + /*
> > + * This function can be called twice in panic path, but obviously
> > + * we execute this only once.
> > + */
> > + if (cpus_stopped)
> > + return;
> > +
> > + cpus_stopped = 1;
> > +
> This cpus_stopped=1 can't happen on multiple CPUs at the same time as any
> second
> call is guaranteed to be on the same CPU, both are behind panic()s
> 'atomic_cmpxchg()'.
'cpu_stopped' variable is not for the race of multi CPUs.
This variable is simply to prevent from calling
'smp_cross_call(&mask, IPI_CPU_CRASH_STOP)' twice in the
machine_crash_shutdown().
Please look at following call path.
* crash_kexec_post_notifiers : true
panic()
crash_smp_send_stop() {
...
cpu_stopped = 1 <= make it '1'
smp_cross_call() <= save crash dump for nonpanic cores
}
__crash_kexec()
machine_crash_shutdown()
crash_smp_send_stop() {
if (cpu_stopped)
return <= just return.
}
>
>
> Other than my '/proc/vmcore is not available' question above, this looks fine
> to me:
> Reviewed-by: James Morse <james.mo...@arm.com>
> Tested-by: James Morse <james.mo...@arm.com>
>
>
> Thanks!
>
> James
>
>
>
